This invention relates to a synchronizing signal generator for generating a synchronizing signal surely synchronized with the desired signal.
Synchronizing signal generators of the type referred to have the function that the input signal applied, as a reference signal, thereto is directly or indirectly compared with an oscillating signal produced by the local oscillator and the oscillator is controlled within the feedback loop for phase synchronization so as to render a difference in frequency between both signals while one portion of the oscillating signal thus controlled is delivered as the synchronizing signal. The input signal includes the requisite signal with which the synchronizing signal to be delivered is synchronized and a plurality of unrequisite signals dispersedly located about the requisite signals.
When the feedback loop has a narrow loop band width, the local oscillator might be erroneously locked in a phase of any of the unrequisite signals.
In order to avoid this erroneous locking of the local oscillator, it has been previously proposed to connect a variable attenuator to the feedback loop so that the input signal is first attenuated enough to have a voltage lower than a threshold value of a phase detector included in the feedback loop. Then the attenuation of the input signal is gradually decreased until the local oscillator is locked in the phase of the requisite signal. This measure has been inevitably disadvantageous in that, with loop constants varied, the pull-in operation is apt to be unstable because the requisite and unrequisite signals are applied to the feedback loop with their low levels due to the similar attenuation of both signals.
In order to eliminate thus disadvantage, it has been already proposed to substitute a white noise generator and a noise control for the variable attenuator. White noise from the white noise generator is controlled by the noise control to have its level equal to or higher than levels of the unrequisite signals but lower than that of the requisite signal. The white noise thus contrtolled in level is added to the input signal to mask the unrequisite signals. Then the input signal with the white noise is applied to the feedback loop for phase synchronization having a wide loop band width to lock the local oscillator in the phase of the requisite signal, whereupon the white noise is prevented from being added to the input signal. This measure has been disadvantageous in that, even though the local oscillator would have been locked in the phase of the requisite signal, an output developed in an synchronizing signal output terminal causes the phase jitter due to any of the unrequisite signal entering the wide loop band after the removal of the white noise.
Accordingly it is an object of the present invention to provide a new and improved synchronizing signal generator for generating a synchronizing signal with a high S/N ratio through the synchronization with the requisite signal included in an input signal having added thereto unrequisite or noise signals at levels as required and the removal of the noise signals after the synchronization.
It is another object of the present invention to provide a new and improved synchronizing signal generator for generating a synchronizing signal with a high S/N ratio synchronized with the requisite or object signal lying within a wide frequency range and in the stabilized operating state.